1. Technical Field
This invention relates generally to a method for stabilizing the electron multiplication gain of a photo-detector, such as a photomultiplier tube (PMT), and, more specifically, to the use of such photo-detector stabilization method in a reading device that measures the light emitted from luminescent phosphors, for example as used to determine the level of ionizing radiation to which the phosphor has been exposed.
2. Discussion of Prior Art
Radiation dosimeters are widely used to measure the presence of ionizing radiation. Various radiation dosimeter means and methods are used to detect radiation, and among these are electronic device response, photographic film response, and light emission from luminescent phosphors. The emission of light from luminescent phosphors may be stimulated thermally or optically. When luminescent phosphors are used for radiation dosimetry, the amount of luminescent emission is directly related to the ionizing radiation exposure to which the phosphor has been subjected. Thus to obtain an accurate measurement of radiation exposure, emitted light must be measured accurately.
Thermally stimulated and optically stimulated phosphors are used extensively in radiation dosimeters to monitor radiation exposure levels, and are well known in the art. The mechanism of phosphor response to ionizing radiation and the luminescence that results under thermal or optical stimulation is well understood, and has been described by others, such as Huston et al. in U.S. Pat. No. 6,087,666 and McKeever et al. in U.S. Pat. No. 5,892,234.
When a luminescent radiation dosimeter has been exposed to ionizing radiation, the exposure level can be determined by stimulating the phosphor and comparing the resulting light emission to the light emission obtained for a known radiation exposure. Methods and means of radiation dosimetry utilizing thermally stimulated luminescence and optically stimulated luminescence have been described by others, as described and referenced by McKeever et al. in U.S. Pat. No. 5,962,857.
When used in radiation monitoring badges worn by personnel, luminescent phosphors are typically in the form of individual dosimeter elements, which are comprised entirely or substantially of the luminescent phosphor. To obtain a reading of the radiation exposure which a radiation monitoring badge has received, the individual dosimeter elements are stimulated and the resulting luminescent emission is measured and compared to a calibrated radiation response. This measurement is performed in a luminescent phosphor dosimeter reader that includes phosphor stimulating means and a photo-detector to measure the light emitted by the luminescent phosphor.
Since emitted light levels from luminescent phosphors used for radiation dosimetry are typically very low, the photo-detector used to detect this light must be capable of measuring very low light levels. At the most basic level of operation for an electronic photo-detector, a single photon of light incident on the photo-detector photosensitive material may produce a single free electron for detection. Direct detection of small numbers of electrons is difficult, thus a photo-detector for low light levels typically employs an electron multiplication means to increase the number of electrons to be detected. Such means for electron multiplication are found in detectors for low level light such as a photomultiplier tube (PMT), a micro-channel plate consisting of an array of very small photomultiplier tubes, and an avalanche photodiode (APD). The electron multiplication gain of APDs is on the order of 1000, much lower than that of PMTs which have a typical gain on the order of 1,000,000. Thus a PMT is the preferred photo-detector for low level light detection.
In order to measure the emitted light accurately, a photo-detector must be calibrated, and this calibration must be maintained during photo-detector operation. Stabilizing the photo-detector environment, principally the temperature, is helpful for maintaining calibration. Calibration may also be maintained by measuring the photo-detector performance and correcting or compensating variations. Often both approaches are used in combination to maintain calibration of a light measurement system.
The electron multiplication gain is the primary factor affecting the output response stability of a photo-detector such as a PMT which utilizes electron multiplication, and since this gain is very sensitive to temperature, one of the most common stabilization methods is to maintain the photo-detector at a constant temperature. Photo-detectors are often temperature stabilized for this reason, generally at temperatures below ambient to reduce electronic noise and obtain better performance. The use of a thermoelectric cooler for the temperature stabilization of a PMT is described by Robertson et al. in U.S. Pat. No. 3,925,665. Alternatively, Rozsa in U.S. Pat. No. 6,407,390 presents temperature compensation circuitry that reduces temperature effects without stabilizing the actual temperature of the photo-detector. Other types of environmental mitigation are also used along with temperature stabilization, such as shielding against electrical and magnetic fields.
Calibration is also frequently maintained by a capability to measure photo-detector performance, so that performance variations can be compensated or corrected. Robertson et al. in U.S. Pat. No. 3,925,665 describes the use of a light source to check the sensitivity of a light measurement arrangement and stabilize the gain in a reader for thermoluminescence dosimetry. Brum et al. in U.S. Pat. No. 4,727,253 similarly refers to the use of a reference light source in a thermoluminescent dosimetry system to maintain calibration. A reference light source is used to ensure correct and accurate operation in many light measurement applications, especially where the photo-detector is a PMT. To be effective for this purpose, a reference light source must be stable, with a fixed output intensity and wavelength.
The performance of a photo-detector system is often measured using a reference light source which is inherently stable or is made stable by associated stabilization means. A stable reference light permits the performance of a PMT light measurement arrangement to be checked and adjusted, especially to correct or compensate changes in the PMT gain. The PMT is exposed to the light source and the PMT output signal is measured and compared to the light source calibration value. The PMT gain is then increased or decreased, as necessary, to obtain a measured light source reading that corresponds to the calibration value. Alternatively, light measurement calibration may be performed by applying a correction factor, calculated as the ratio of the light source calibration value to the measured light source reading, to the PMT signal or data.
Two common light sources used as reference sources include a scintillator material activated by radioactive decay particles or high energy photons, and a light emitting diode (LED). Since radioactive decay is not affected by temperature, it can be used with a scintillator to produce a highly stable light source. If the decay particles have high energy, flashes of light containing many photons will be produced. Mattern in U.S. Pat. No. 5,610,396 describes the use of a gamma radiation source incident upon a scintillator to generate light flashes for calibration. In contrast, carbon-14 decay produces individual photons in a scintillator, and Valenta in U.S. Pat. No. 5,321,261 describes the use of a scintillator excited by carbon-14 that is used as a calibration light source. Light sources containing radioactive materials are undesirable, however, because radioactive materials are hazardous, and thus their use is greatly restricted. A scintillator light source utilizing a stabilized ultraviolet source for stimulation is described by Kimmich et al. in U.S. Pat. No. 6,087,656.
A light emitting diode (LED) is very convenient for use as a reference light source. The output level and wavelength of an LED, however, are temperature dependent, and thus the LED must be stabilized in order to be used as a reference light source. To obtain a stable output, an LED reference light typically includes temperature stabilization to obtain a stable LED wavelength, and circuitry to provide closed-loop control of the LED output intensity based on measurement of LED output by a photodiode, or other similar photo-detector used exclusively for measuring the LED output. The use of an LED as a reference light for adjusting gain is described by Kobayashi in U.S. Pat. No. 5,079,424. Taylor in U.S. Pat. No. 5,715,048 claims a stable LED light source for calibration based on the use of a photo cell and feedback control circuitry to stabilize the LED output. Brown et al. in U.S. Pat. No. 5,859,429 describes a similar arrangement as a check source, identifying a blue LED as the source and a photodiode as the detector used to adjust the LED intensity. In some cases LED temperature stabilization may be provided by the PMT temperature stabilization means, since the LED is typically in close proximity to the PMT. Whatley in U.S. Pat. No. 4,220,851 describes the temperature stabilization of an LED light source used to stabilize PMT gain.
The LED light source claimed by Whatley for gain stabilization provides flashes of light. The use of light flashes for calibration from a lamp is described by Ried et al. in U.S. Pat. No. 3,515,878. Light flashes can also be produced by radioactive decay events in a light source containing radioactive material. When gain measurements are performed based on flashes of light, gain adjustment may be based not on the output obtained from a single flash, but on the average output of a number of flashes, or on the distribution obtained from many flashes using a pulse height analyzer, as described by Ried et al. in U.S. Pat. No. 3,515,878 and Parker in U.S. Pat. No. 4,322,617. The output signal obtained from a flash of light consisting of a large number of photons appears as a large pulse. If pulses vary in size due to the intrinsic nature of the flashes and for other reasons, measuring and correcting gain based on a distribution of these pulses, rather than a pulse average, can provide better calibration accuracy.
Gain measurements can be made apart from the use of a reference light source. A method for PMT gain stabilization is claimed by Nurmi et al. in U.S. Pat. No. 5,548,111, in which the gain is calculated as the ratio of the anode current to the photocathode current, and the PMT voltage is adjusted to keep this ratio constant. This is similar to the method of Oikari et al. in U.S. Pat. No. 5,157,250 in which the PMT gain is stabilized by maintaining a constant ratio between the anode current and the first dynode current.
The various means and methods described above for stabilizing the response of a PMT photo-detector are effective to varying degrees and some are widely used. In general, they result in a photo-detector system that is significantly larger, more complex and more expensive than the photo-detector itself, thus restricting the use of these photo-detectors for some applications.